From Snom User Wiki

The phone will subscribe to the presence state of the destination url with event type presence. The associated led will reflect the presence state of the destination e.g. ringing, available etc. Hitting the programmable key (usually when the destination is available and can receive a call) shall dial that number.

Note

Your VoIP PBX must support this functionality. If the standard implementation of this feature does not comply to your PBX, you can define your own customized model by using Function Keys, option "XML Definition"